cmd/go,testing: re-implement testing.Coverage

This patch revives the testing.Coverage() function, which takes a
snapshot of the coverage counters within an executing "go test -cover"
test binary and returns a percentage approximating the percent of
statements covered so far.

Fixes #59590.

Change-Id: I541d47a42d71c8fb2edc473d86c8951fa80f4ab0
Reviewed-on: https://go-review.googlesource.com/c/go/+/495450
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/load/test.go

@@ -932,11 +932,14 @@ func init() {
 func runtime_coverage_processCoverTestDir(dir string, cfile string, cmode string, cpkgs string) error
 
 //go:linkname testing_registerCover2 testing.registerCover2
-func testing_registerCover2(mode string, tearDown func(coverprofile string, gocoverdir string) (string, error))
+func testing_registerCover2(mode string, tearDown func(coverprofile string, gocoverdir string) (string, error), snapcov func() float64)
 
 //go:linkname runtime_coverage_markProfileEmitted runtime/coverage.markProfileEmitted
 func runtime_coverage_markProfileEmitted(val bool)
 
+//go:linkname runtime_coverage_snapshot runtime/coverage.snapshot
+func runtime_coverage_snapshot() float64
+
 func coverTearDown(coverprofile string, gocoverdir string) (string, error) {
 	var err error
 	if gocoverdir == "" {
@@ -957,7 +960,7 @@ func coverTearDown(coverprofile string, gocoverdir string) (string, error) {
 
 func main() {
 {{if .Cover}}
-	testing_registerCover2({{printf "%q" .Cover.Mode}}, coverTearDown)
+	testing_registerCover2({{printf "%q" .Cover.Mode}}, coverTearDown, runtime_coverage_snapshot)
 {{end}}
 	m := testing.MainStart(testdeps.TestDeps{}, tests, benchmarks, fuzzTargets, examples)
 {{with .TestMain}}

src/cmd/go/testdata/script/testing_coverage.txt

@@ -0,0 +1,57 @@
+
+# Rudimentary test of testing.Coverage().
+
+[short] skip
+[!GOEXPERIMENT:coverageredesign] skip
+
+# Simple test.
+go test -v -cover -count=1
+
+# Make sure test still passes when test executable is built and
+# run outside the go command.
+go test -c -o t.exe -cover
+exec ./t.exe
+
+-- go.mod --
+module hello
+
+go 1.20
+-- hello.go --
+package hello
+
+func Hello() {
+	println("hello")
+}
+
+// contents not especially interesting, just need some code
+func foo(n int) int {
+	t := 0
+	for i := 0; i < n; i++ {
+		for j := 0; j < i; j++ {
+			t += i ^ j
+			if t == 1010101 {
+				break
+			}
+		}
+	}
+	return t
+}
+
+-- hello_test.go --
+package hello
+
+import "testing"
+
+func TestTestCoverage(t *testing.T) {
+	Hello()
+	C1 := testing.Coverage()
+	foo(29)
+	C2 := testing.Coverage()
+	if C1 == 0.0 || C2 == 0.0 {
+		t.Errorf("unexpected zero values C1=%f C2=%f", C1, C2)
+	}
+	if C1 >= C2 {
+		t.Errorf("testing.Coverage() not monotonically increasing C1=%f C2=%f", C1, C2)
+	}
+}
+

src/testing/cover.go

@@ -47,6 +47,9 @@ type Cover struct {
 // It is not a replacement for the reports generated by 'go test -cover' and
 // 'go tool cover'.
 func Coverage() float64 {
+	if goexperiment.CoverageRedesign {
+		return coverage2()
+	}
 	var n, d int64
 	for _, counters := range cover.Counters {
 		for i := range counters {

src/testing/newcover.go

@@ -15,16 +15,18 @@ import (
 // cover2 variable stores the current coverage mode and a
 // tear-down function to be called at the end of the testing run.
 var cover2 struct {
-	mode     string
+	mode        string
-	tearDown func(coverprofile string, gocoverdir string) (string, error)
+	tearDown    func(coverprofile string, gocoverdir string) (string, error)
+	snapshotcov func() float64
 }
 
 // registerCover2 is invoked during "go test -cover" runs by the test harness
 // code in _testmain.go; it is used to record a 'tear down' function
 // (to be called when the test is complete) and the coverage mode.
-func registerCover2(mode string, tearDown func(coverprofile string, gocoverdir string) (string, error)) {
+func registerCover2(mode string, tearDown func(coverprofile string, gocoverdir string) (string, error), snapcov func() float64) {
 	cover2.mode = mode
 	cover2.tearDown = tearDown
+	cover2.snapshotcov = snapcov
 }
 
 // coverReport2 invokes a callback in _testmain.go that will
@@ -46,3 +48,12 @@ func coverReport2() {
 func testGoCoverDir() string {
 	return *gocoverdir
 }
+
+// coverage2 returns a rough "coverage percentage so far"
+// number to support the testing.Coverage() function.
+func coverage2() float64 {
+	if cover2.mode == "" {
+		return 0.0
+	}
+	return cover2.snapshotcov()
+}